How can DNA be so long when the cells are so small?
(Lansing State Journal, November 17,1993)


It seems amazing, but it is true.  The DNA in every one of your cells, if placed end to end, would have a length of 990 centimeters. But there is not reason to be amazed that it all fits into a tiny cell.  A simple calculation of the volume of DNA makes it clear that there is quite a bit of space for it in every cell.  Now, cells are not all the same size or shape and DNA is not simply a long helical string floating in the nucleus.  Nonetheless, we can make the following approximate calculation.

Consider a human cell to be a very small square box.  On average it will have sides with lengths of about 10 microns.  (A micron is 10-6 meters).  This is reasonable, and can be verified by directly measuring cells under a microscope.  The volume of such a cube would be 10-15 cubic meters.  Now, DNA can be modeled as a cylinder with a radius of 1 nanometer (10-9 meters).  We know that this is more or less correct, because studies of DNA using X-ray spectroscopy show that DNA forms long thin helixes of measurable dimensions.  If the length of the DNA is really .99 meters, then its volume would be 3.1x 10-18 cubic meters.  That means that DNA takes up less than 1 percent of the cell's volume!

In fact, a person's DNA is split up into separate pieces called chromosomes.  And these chromosomes are kept in a special compartment in the cell called the nucleus.  The nucleus itself may take up to 10 percent of the cell's volume, because it contains a variety of proteins and RNA as well as DNA.

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